yup, what plutom said for the centaur is easiest way.

 

Any modelling program that makes an obj file will work to build props or figures for Poser. There are several free ones (blender, wings), or you can try hexagon, shade, silo, Cinema 4d , Lightwave, 3ds max etc depending on how much money you want to spend.  You dont need a lot of frills if you are aiming to use the model in Poser.